In the past, various different methods have been utilized to secure arcuate shaped magnet material elements formed of a frangible material to a circumferential surface of a rotor core. In one of these past methods, the rotor core was provided with protrusions which were associated in displacement preventing engagement with the arcuate magnet material elements thereby to maintain them in place seated against the circumferential surface of the rotor core. In another of the past methods, a fiber, plastic or metallic wrap or sleeve was provided enveloping at least the arcuate outer surfaces of the magnet material elements thereby to maintain the magnet material elements against displacement from their seated engagement with the circumferential surface of the rotor core.
In still another of the aforementioned past methods of securing arcuate magnet material elements to a rotor core, a hardenable adhesive material was applied to at least one of the circumferential surface of the rotor core and the arcuate inner surfaces of the magnet material elements. After such application of the hardenable adhesive material, the arcuate inner surfaces of the magnet material elements were seated or abutted against the circumferential surface of the rotor core, and in this position, the permanent magnet material elements and the circumferential surface of the rotor core were merely clamped together until the hardenable adhesive material set or hardened thereby to adhere together the magnet material elements and the circumferential surface of the rotor core. Since both the arcuate inner surfaces of the magnet material elements and the circumferential surface of the rotor core may have high points or areas due to tolerance variations during the manufacture thereof, the arcuate inner surfaces of magnet material elements and the circumferential surface rotor core were at least in part in surface-to-surface engagement with each other; due at least in part to such aforementioned surface-to-surface engagement, it is believed that one of the disadvantageous or undesirable features of this past method was that some of the frangible arcuate magnet material elements fractured or cracked when pressure or force was applied thereto to clamp them against the rotor core. Since the high points or areas on the arcuate inner surfaces of the magnet material elements and the circumferential surface of the rotor core effected by the aforementioned tolerance variations were clamped directly into the aforementioned surface-to-surface engagement another disadvantageous or undesirable feature of this past method is believed to be that dependable, uniform bond strengths of the hardenable adhesive material could not be attained between the arcuate inner surfaces of the magnet material elements and the circumferential surface of the rotor core. Due to this nonuniformity of bond strength, it is also believed that the rotor assemblies fabricated by this past method may have been acceptable for use in low speed motor applications, such as ceiling fans for instance, but might not have been unacceptable in relatively high speed motor applications, such as clothes washers and various hermetic motor applications for instance. Additionally, it is further believed that another disadvantageous or undesirable feature of rotatable assemblies fabricated by the past method was that the outside diameter or circumference thereof was not constant since the radial distance between the circumferential surface of the rotor core and the arcuate outer surface of each permanent magnet material element may have varied due to the aforementioned tolerance buildup therebetween. While the prior art rotatable assemblies believed to have the above discussed disadvantageous feature may have been acceptable for some relatively low speed motor applications, it is believed that they may have been too far out of balance for use in a motor application of relatively high speeds, say for instance, at least about nine thousand revolutions per minute (9000 rpm) or above. Also with respect to the prior art rotatable assemblies believed to have the above discussed disadvantageous feature, it is believed that a flux gap between a stator bore and the outer arcuate surfaces of the permanent magnet material elements on such prior art rotatable assemblies may have been deleteriously affected. Furthermore, it is also believed that another disadvantageous or undesirable feature of rotatable assemblies manufactured by this past method was that the arc lengths of the permanent magnet material elements may not have been evenly distributed about the circumferential surface of the rotor core thereby also to effect a too far out of balance rotatable assembly for use in the aforementioned higher speed motor applications.